U.S. Pat. No. 6,783,409 discloses a pod unit including a housing and an assembly unit, a so called seating, for the pod housing. The housing includes a lower casing portion incorporating the propulsion unit, an intermediate portion comprising air ducts, and an upper portion being fixedly connected to a steering engine with a gear ring being fixed at the base. The seating has a lower portion and an upper portion as well as a base for the installation of the pod housing. The lower portion of the seating is positioned on the first outer bottom of the vessel and the upper portion of the seating is positioned at the level of the second inner bottom of the vessel. The base is positioned at the lower portion of the seating.
A cooling system having at least one fan is mounted within the seating. The cooling air is directed downwards through an inlet duct in the middle of the seating passing down to the lower portion of the pod housing. Hot air returns from the lower portion of the pod housing via a return duct surrounding the inlet duct. The hot air is directed from the return duct to a cooling unit within the seating and then again back to the inlet duct. Cooling air is thus circulated with one or two fans between the cooling unit in the seating and the lower portion of the pod housing.
U.S. Pat. No. 6,935,907 is a continuation of the above mentioned U.S. Pat. No. 6,783,409 and discloses a similar assembly unit as is disclosed in U.S. Pat. No. 6,935,907.
EP patent publication 0 590 867 discloses a ship propulsion arrangement having a tubular shaft rotatable about a vertical axis and an outer casing at the lower end of the shaft. An electric motor within the outer casing is connected with a shaft to a propeller outside the outer casing. There are webs arranged within the outer casing forming structural elements of the casing. The webs stiffen and support the outer casing, secure the electric motor in place, and form ducts for cooling air directed to the electric motor and for return air directed from the electric motor.
Another known propulsion unit includes a hollow strut having an upper portion and a lower portion. An upper end portion of the upper portion passes through a passage formed between a first outer bottom and a second inner bottom in a vessel. The upper end portion is rotatably supported at a hull of the vessel with a slewing bearing and sealed against the hull of the vessel with a slewing seal. An electric motor is positioned in the lower portion of the hollow strut and a propeller positioned outside the strut is connected to the electric motor. A generator connected to a combustion engine is positioned within the vessel. The generator produces electric energy to the electric motor in the lower portion of the strut. The electric energy is transferred via a slip ring unit from the interior of the vessel to the electric motor in the lower portion of the strut.
The cooling of the electric motor in the lower portion of the strut is done by circulating cooling air from within the interior of the vessel to the electric motor. Cooling air is circulated from the interior of the vessel with a fan to the upper end portion of the upper portion of the strut and further down via the strut to the electric motor and hot air is directed up again via the strut to the cooling unit positioned inside the vessel. The cooling air passing down via the strut to the electric motor is separated from the hot air passing up via the strut by walls forming vertical air ducts within the strut.
The cooling air duct leading cooling air from the cooling unit to the upper end portion of the strut and the return air duct leading hot air from the upper end portion of the strut back to the cooling unit are, in known arrangements, positioned above the slewing bearing. This known cooling arrangement leads to a rather extensive construction. A part of the slewing bearing in this known arrangement is subjected to warm internal air in the interior of the vessel and another part of the slewing bearing is subjected to cold air propagating along the steel structures from the sea. The temperature difference within the slewing bearing creates thermal stresses in the slewing bearing. The thermal stresses can shorten the lifetime of the slewing bearing.